Antimicrobial Activity of Silver-Containing Dressings is Influenced by Dressing Conformability with a Wound Surface
Disclosure: All authors are paid employees of ConvaTec Ltd, Flintshire, United Kingdom.
W ound tissue can provide a favorable environment for microbial colonization with a variety of aerobic and anaerobic bacteria.1 Although wound colonization per se is not an indication of infection, factors, such as diabetes, immunosuppression, or the concomitant administration of certain medications, can influence the bacterial balance,2 which may lead to a wound bioburden greater than the level manageable by the host.3 As a consequence of these conditions, clinical infection and delayed healing may occur.4 As well as increasing pain and discomfort for the patient, a slow healing or infected wound can result in an increased burden on the healthcare provider in terms of cost and time.5
With the growing presence of antibiotic-resistant strains of bacteria, topical antimicrobial agents, such as silver and iodine, have once more come into clinical favor.5–7 Topical silver has broad-spectrum antimicrobial activity that encompasses many antibiotic-resistant wound pathogens.4 Unlike the case with antibiotics, there is little current evidence of emerging microbial resistance to silver.8 Various wound dressings containing silver are now available for the management of critically colonized and locally infected wounds,8 and these dressings differ in structure and physical properties, type and amount of silver contained in the dressing, and the mechanism by which silver is delivered.9
Managing wound infection and reducing the risk of infection are important objectives in wound management. Good clinical practice should include the correct choice of wound dressing to prevent and manage local infections in at-risk wounds.5 Factors to be considered include the ability of the chosen dressing to manage exudate in heavily exuding infected wounds,10 act as an effective antimicrobial barrier,10 and absorb and retain bacteria.10 The appropriateness of the dressing for the size, depth, and location of the wound is also an important factor in dressing selection. Comparatively little is known about how these properties are modified by local factors at the wound interface.
This study examined 2 silver-containing wound dressings, a silver-containing Hydrofiber® (SCH) dressing (AQUACEL® Ag, ConvaTec, Skillman, NJ, USA) and a nanocrystalline silver-containing (NSC) dressing (Acticoat™, Smith & Nephew, London, UK), that are used in the clinical management of wounds at risk of infection. Both have demonstrated highly efficacious antimicrobial activity against wound pathogens in vitro9 including methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant enterococcus (VRE).11–13
Structurally, these dressings differ in their underlying construction and in the mechanisms by which ionic silver is made available. The SCH dressing is made of the hydrocolloid polymer carboxymethylcellulose to which silver ions are attached.14 The dressing fibers absorb wound fluid, swelling to form a soft cohesive gel that covers the wound surface.15 This moist gel mass is kept in direct contact with the wound bed leaving little or no space between the dressing and the wound.16 The ionic silver in the dressing has been shown to provide broad-spectrum antimicrobial activity for up to 14 days.11
In contrast, the NSC dressing consists of 1 layer of a rayon/polyester inner core sandwiched between 2 layers of polyethylene net.17 This net is coated with nanocrystalline silver that is released when water contacts the dressing.18 Studies have shown that silver release from NSC dressings is faster and is delivered more rapidly than from either silver nitrate or silver sulfadiazine dressings.19 The NSC dressing provides an effective antimicrobial barrier for up to 7 days.17
This study used in-vitro models to assess the conformability of SCH and NSC dressings to unevenly contoured wound-like surfaces and to examine the degree to which this correlated with antimicrobial effect against 2 common wound pathogens, Pseudomonas aeruginosa and methicillin-resistant Staphylococcus aureus.
Dressing contact with wound tissue and simulated dry eschar. Small sections of wound tissue obtained from electively amputated lower limbs and simulated dry eschar (human dermis, approximately 10 mm x 4 mm, dried overnight at 37oC) were fixed in a vertical plane on a freshly cleaned microscope slide using a cyanoacrylate adhesive (LOCTITE® 4062 instant adhesive, Loctite, Hertfordshire, UK). The Ethics Committee of North East Wales Trust, Wrexham Maelor Hospital, UK, granted ethical approval to allow human skin to be used from lower limb amputations.
A dry piece of each silver-containing dressing (approximately 10 mm x 4 mm) was then placed carefully onto the upper tissue surface. Glass inserts were placed at either end of the microscope slide to allow a second slide to be placed on top of the tissue and dressing to create a sandwich effect. The ends of the combined microscope slides were then clamped together using bulldog clips, allowing the slide to be placed horizontally onto a microscope stage (Wild Heerbrug, Germany). Images were captured using an attached digital camera (Polaroid DCM 1e, Polaroid Corporation, Waltham, Mass, USA).
Once the dry images had been captured, water was added to each dressing to ensure hydration to saturation point without causing a visible fluid leakage. The volumes used were up to a maximum of 100 µL for the NSC dressing and up to 400 µL for the SCH dressing. Water was added through the gap between the slides to allow observation of the dressing/tissue interaction in the hydrated state.
Dressing contact with an inoculated indented agar surface. Standard reference strains of methicillin-resistant Staphylococcus aureus (MRSA) (NCTC 12232) and antibiotic-resistant Pseudomonas aeruginosa (NCTC 8506) were used to investigate the effect of dressing conformability on antimicrobial efficacy for a SCH dressing and a NSC dressing.
An agar plate model was developed to investigate dressing conformability, ie, the degree to which each dressing maintained intimate contact with a surface-inoculated agar plate during a specified time period. Nonwoven, fabric-folded 4 cm x 4 cm swabs (Topper 8, Johnson & Johnson, Somerville, NJ, USA) were aseptically transferred to the center of standard, pre-dried tryptone soy agar plates (TSA, Lab M) and pressed onto the agar to enable direct contact without damage to the agar surface. A 15-mL volume of molten TSA (precooled to approximately 40oC) was then poured over each swab-impregnated plate to create a second 2–3 mm layer of agar. All agar plates prepared in this way were allowed to solidify overnight under aseptic conditions. The gauze dressings were then removed aseptically from each agar plate, leaving an impression (4 cm x 4 cm x 2–3 mm depth) within the agar surface (Figure 1).
Challenge organisms were cultured on separate TSA plates and incubated aerobically for 24 hours at 35oC (± 3oC). After this period, discrete colonies of each strain were selected and suspended separately in a 10-mL volume of maximal recovery diluent (MRD, Lab M) to yield a concentration of approximately 1 x 104 cfu/mL. A sterile swab was then used to inoculate each organism across the entire surface of the agar plates, including each indented area. All inoculated agar plates were then incubated aerobically at 35oC (± 3oC) for 4 hours to allow the formation of microcolonies.
Following incubation, 5 cm x 5 cm squares of each test dressing were aseptically transferred to separate indented agar plates inoculated with either MRSA or P. aeruginosa by gently placing the dressing over the impression created within the agar. Careful positioning ensured that dressing edges overlapped the edge of the impression by approximately 1 cm. Each dressing was then hydrated to saturation point as previously described with either sterile water or saline. The volumes required for hydration of the 5 cm x 5 cm samples were 4 mL for the SCH dressing and 1.2 mL for the NSC dressings (1.2 mL reflecting the significantly lower absorption rate of the latter dressing).
Triplicate agar plates were prepared for each challenge organism, test dressing, and hydration method. A negative control plate (containing no dressing) was also prepared for each challenge organism to determine viability. All dressing-containing agar plates (including negative controls) were then re-incubated at 35oC (± 3oC) for a further 24 hours. Each dressing was then removed to allow examination of the effect of dressing conformability on microbial activity (ie, growth/no growth) within the agar impression directly beneath the test dressing.
Image analysis software (ImageTool for Windows, version 3.0, The University of Texas Health Science Center in San Antonio, San Antonio, Tex, USA) was then used to determine the extent of bacterial growth beneath each test dressing within the agar impression. Initially, the surface area (mm2) of the total swab impression within each agar plate was recorded, and the mean of 2 measurements was calculated for each plate. Further measurements of the areas of bacterial growth beneath each test dressing were then made; the mean of 3 measurements was then calculated for each agar plate. Measurements for the negative control agar plates were also taken (n = 2). These data were then used to determine the percentage of bacterial growth beneath the test dressings within the agar impressions.
Dressing conformability with wound tissue. Viewed under a light microscope, the prepared slides showed that the hydrated SCH dressing formed a cohesive gel that conformed closely to the wound surface (Figure 2A). The NSC dressing did not conform to the contours of the wound tissue as effectively, and small areas of noncontact were clearly visible (Figure 2B).
Dressing conformability with dry eschar. The SCH dressing showed good conformability with dried human dermis (simulated dry eschar). These observations were evident following the addition of volumes of distilled water sufficient to hydrate each dressing to saturation point (ie, up to 400 mL for the SCH dressing [Figures 3A and 3B] and 100 mL for the NSC dressing [Figure 4A]). Small areas of noncontact were visible with the NSC dressing (Figure 4A), and these remained after additional pressure was applied to improve the dressing contact (Figure 4B).
Contact with an inoculated indented agar surface. Figures 5 (A to D) and 6 (A to D) show TSA plates inoculated with MRSA and P. aeruginosa, respectively. Visual comparison of the plates showed that the impressions that had been covered by the SCH dressing were almost entirely clear of bacteria, irrespective of whether the plates were inoculated with MRSA (Figures 5A and B) or P. aeruginosa (Figures 6A and B). In addition, most of the plates to which the SCH dressing was applied showed some evidence of extended antimicrobial effect that was associated with peripheral fibers around the dressing (Figure 5A). In contrast, significant areas of bacterial growth were apparent in the impressions covered by the NSC dressing.
Image analysis showed that for water-moistened dressings on plates inoculated with MRSA, 94.86% of the area under the SCH dressing was clear of bacterial growth at the conclusion of the experiment, compared with only 6.25% of the area under the NSC dressing (control: 0%). In clinical use, the NSC dressing should be moistened with water rather than saline (as indicated in the manufacturer’s instructions for use), but in the present experiments, the differences between the 2 wound dressings were comparable regardless of whether water or saline was used. The differences were also similar in magnitude for both MRSA and P. aeruginosa (Figure 7). These differences in antimicrobial efficacy correlate with the differences in dressing conformability seen in the wound tissue experiments.
The results of this study indicate that the SCH dressing is more effective at killing bacteria inoculated onto a simulated wound surface with uneven contours than the NSC dressing. Both visual and image analysis showed significant differences in bacterial growth in agar indentations covered with SCH dressings compared to those under NSC dressings. These differences were similar regardless of whether MRSA- or P. aeruginosa-inoculated plates were assessed.
The dressing conformability assessments reported here show that the cohesive gel formed following hydration of the hydrofibers within the SCH dressing15 is capable of maintaining close contact with both wound tissue and simulated dry eschar. However, the NSC dressing was less conformable, and areas of noncontact were visible between the dressing and the wound tissue, even when additional gentle pressure was applied to improve contact.
The efficacy of antimicrobial dressings, such as those containing silver, would be expected to be dependent primarily on the availability of the active agent in these dressings. In the case of the NSC dressing, metallic nanocrystalline silver particles are released from the dressing and transition to active ionic silver (Ag+) in a hydrated environment. With the SCH dressing, ionic silver is readily available following hydration of the hydrofibers.
If antimicrobial activity was dependent only on the availability of silver within the dressing, the dressings would have been expected to perform equally well in the authors’ in-vitro model. The fact that the SCH dressing generally performed more effectively than the NSC dressing indicates that other factors played a role in the overall antibacterial effect.
It is important to note that TSA was used as the bacterial culture medium throughout these experiments. This medium contains protein and a physiological level of chloride ions, both of which are likely to have influenced the overall results because of the affinity of ionic silver with these components.
Managing the bacterial balance of a wound is important to help prevent infection and encourage wound healing. White et al.5 have pointed out the role of colonized and infected wounds in acting as reservoirs for resistant strains of bacteria and the risk of cross-infection within healthcare environments. To be effective across a wide range of patients, the antimicrobial activity of silver-containing barrier dressings needs to be maintained across wounds of all shapes and sizes, from uneven, shallow leg ulcers to deep pressure ulcers.
The present study demonstrates the effective bactericidal activity of a SCH dressing against antibiotic-resistant strains of 2 common wound pathogens—P. aeruginosa and S. aureus (MRSA). The ability of antimicrobial dressings to effectively control pathogens, such as MRSA, across a variety of wound types is an important step in infection control.12
While both silver-containing dressings are known to be effective at killing wound pathogens including MRSA in vitro,11,13 the present study indicates that antimicrobial effectiveness in the clinical setting may be influenced by the physical properties of the dressings themselves.
These findings suggest that the SCH dressing is likely to provide more widespread antimicrobial protection at the wound-dressing interface than the NSC dressing studied here. This difference can be attributed to variations in dressing design and in the ability to conform well to wound surfaces. This study suggests that a variety of factors are involved in ensuring the most effective use of silver-containing dressings for the management and prevention of wound infection.